As types of electric vehicles, there are known a vehicle including only an electric motor as a drive source, and a hybrid vehicle including an electric motor and an engine as drive sources. The electric vehicle of any of those types includes a battery serving as an electricity storage device for supplying power to the electric motor, and the battery is required to be charged from the outside when the remaining capacity of the battery is low. Further, in the hybrid vehicle including the electric motor and the engine as the drive sources, in normal, the battery is charged by driving the engine. Note that, the battery is sometimes charged by supplying power from an external power supply without driving the engine.
In the electric vehicle including the electric motor as described above, an in-vehicle charger is mounted so as to enable charging of the battery with use of a household commercial power supply as the external power supply. The in-vehicle charger is configured to increase the voltage of the commercial power and convert the commercial power into DC power. In recent years, the electric vehicles such as the EV and the PHEV have become popular. As a result, the in-vehicle charger is desired to be reduced in size and cost by automobile manufacturers, and is desired to be increased in charging efficiency for reduction in battery charging time by users.
Further, the battery inside the electric vehicle is charged by the in-vehicle charger from the household commercial power supply via a public power network, and hence it can be said that the vehicle and the home environment are integrated. Therefore, as the electric vehicle becomes more popular, reliability and quality maintenance are required in both environments of electromagnetic compatibility (EMC) testing for electric vehicles and EMC testing for consumer devices associated with the public power network. Therefore, in such a case, the EMC regulations on the in-vehicle charger are stricter than those of general electrical components.
Note that, the in-vehicle charger generally includes an AC/DC converter and an isolated-type DC/DC converter (hereinafter referred to as “isolated DC/DC converter”). Further, in order to reduce the size and cost of the in-vehicle charger, reduction in size of magnetic components such as a transformer and a reactor is essential, and a higher switching frequency is desired. However, high frequency drive may cause problems such as increase in recovery loss of a diode and increase in surge voltage. In particular, in the case of the in-vehicle charger, a high-voltage battery is connected to the output side of the isolated DC/DC converter. Therefore, the surge voltage generated on the secondary side of the transformer is increased, which leads to fears of increase in withstanding voltage, increase in loss, and EMC deterioration of the element. Therefore, there is a demand for suppression of a surge voltage generated in a secondary-side rectifier circuit of the isolated DC/DC converter.
In view of this, as the first related art, there is known a DC/DC converter for suppressing the surge voltage by including an RCD snubber circuit (for example, see Patent Literature 1). Further, as the second related art, there is known a DC/DC converter for suppressing the surge voltage without including the RCD snubber circuit (for example, see Patent Literature 2).